On the involvement of palladium nanoparticles in the Heck and Suzuki reactions

We describe two different pathways by which palladium-catalyzed Heck and Suzuki coupling reactions take place. When sol-gel entrapped palladium acetate was used as a catalyst, the reactions proceed by the formation of metallic nanoparticles. Such particles were already formed during the immobilization of the palladium salt. Upon application of palladium derivatives of proline, tyrosine, or alanine, either in their homogeneous or in their sol-gel entrapped version, no Pd0 particles could be detected, even when the couplings were carried out in boiling mesitylene (161 °C) or during prolonged reaction periods (48 h). The reactions in the presence of the amino acid derivatives proceeded at a higher rate than those by the entrapped palladium acetate. The existence of palladium nanoparticles was investigated with the aid of high-resolution scanning transmission electron microscopy equipped with energy dispersive X-ray spectroscopy, as well as by X-ray photoelectron spectroscopy.